An electric system including at least a pair of battery strings and an ac source minimizes the use and maximizes the efficiency of the ac source by using the ac source only to charge all battery strings at the same time. Then one or more battery strings is used to power the load while management, such as application of a finish charge, is provided to one battery string. After another charge cycle, the roles of the battery strings are reversed so that each battery string receives regular management.
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1. In an electric system for supplying uninterrupted electric power to a load, the system having at least two battery strings and a primary ac power source, a method of optimally managing the battery strings and using the primary source efficiently comprising:
powering the load and charging all battery strings with the primary ac source; managing one battery string while the load is powered by at least one other charged battery string and without using the primary ac source; powering the load with at least one battery string, without using the primary ac source; powering the load and charging all battery strings using the primary ac source; and managing one of the other battery strings while the load is powered by at least one remaining battery string and without using the primary ac source.
13. An electric system comprising:
an ac source; a load selectively coupled through a switch to said ac source; at least a pair of battery strings connected in parallel, each battery string having a first terminal and a second terminal, the second terminals being each connected to common negative; a bi-directional ac-dc converter having an ac port selectively connected to receive electricity from said ac source or to provide an ac output to said load; and a dc port selectively connected to receive a dc input from either of the first terminals of said battery strings or to provide a dc output to either of said first terminals; a dc converter having an input connected to selectively receive power from either one of said battery strings and an output selectively connected to the first terminal of said other battery string to provide a finish charge said battery string; and wherein said system can selectively charge all battery strings with the ac source; power the load from at least one battery string; and finish charge one battery string while at least one other battery string provides the finish charge and powers the load.
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The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation.
(Not Applicable)
When rechargeable storage batteries are used in electric systems, the requirements for power to be delivered to the connected loads in discharge and/or the availability of power for charging typically do not have values that allow maximization of life of the batteries and maximization of the performance of the systems of which the batteries are a part.
The generic electric system shown in
In
The electrical power requirements of the load(s) and the capabilities of the AC source in electric systems are often such that the battery cannot be charged and/or discharged in the manner required to maximize both the life of the batteries and the performance of the electric system.
For example, batteries based on the zinc/bromine chemistry need to be completely discharged occasionally to maximize their utility. However, such batteries should never be completely discharged when they are used to provide back-up power for critical manufacturing processes, as they otherwise might not be available at critical times.
As another example, certain types of nickel/cadmium batteries exhibit a memory effect which results in an apparent loss of available capacity when repeatedly partially discharged and then recharged. This loss of capacity can be recovered by completely discharging and recharging the battery. These batteries are sometimes used in hybrid electric vehicles where power for recharge is only available during vehicle operation, so the frequent complete discharges these batteries require for optimal life and performance cannot be effected.
As a third example, the state of charge of lead-acid batteries used to help match supply from an electric generator (or electric generators if they are connected into an electricity supply network) and the demand (load requirements) from customers connected to that generator, cannot be optimally managed because the power available for recharge or that required in discharge are determined by the difference between the supply available from the generator(s) and the load demanded by customers. Lead-acid batteries perform best and live longest if each charge is completed (finished) properly and if they are not discharged too deeply. On the other hand, system performance will be maximized if the generator is used only when absolutely necessary. None of these optimization criteria can be strictly adhered to because of the highly variable power available or required in the supply-load matching process. Finishing charge and avoiding overdischarge of lead-acid batteries, and optimizing the performance of systems using lead-acid batteries to help generators match supply and demand, are one of the most important potential applications for the invention disclosed herein.
Most battery manufacturers offer guidelines for ways to optimally charge and discharge their products so as to maximize life and performance. Implementation of these guidelines is made complicated for users by virtue of the fact that most batteries are in fact a collection of individually manufactured units, each of which has slightly different performance characteristics.
The most fundamental unit of batteries is a cell, a unit of 1 to 4 volts depending on the chemistry on which the battery is based. A cell consists of a collection of positive electrodes in parallel and negative electrodes in parallel, juxtaposed so to provide the power and the ampere-hour capacity specified. Sometimes, a few cells (of the order of three to eight) are assembled into modules, with the series electrical connections between the cells being effected during manufacture. Cells or modules are then electrically connected in series at the point of use to form strings.
Other batteries are based on cell-stacks consisting of a number of cells electrically connected in series. In this case, modules are fabricated by connecting a number of cell-stacks in electric series and/or parallel. With some battery chemistries, battery auxiliaries, such as pumps for flow batteries or thermal management hardware for both conventional and advanced batteries, are incorporated with the cells or the cell-stacks into a module.
A storage battery (sometimes referred to as a battery system) consists of a number of cells or modules arranged in series and/or parallel arrays. Cells or modules connected in series are collectively referred to as a battery string. Battery strings may then be electrically paralleled. Occasionally, there is only one module in a string, and infrequently, cells are placed only in parallel in a battery. In these cases, there is no meaning to the term string, but if there were, a string would consist of one module or one cell. Alternatively, a battery consists at times of only one string of cells or modules. As discussed here and after, the current invention does not relate to such single string batteries; at least two strings in parallel are required for operation of this invention, although each of the strings may consist of one or more cells or modules in series.
The number of cells or modules in series in a string or battery is determined by the voltage desired for the battery system, which is in turn set by the requirements of the charging and discharging equipment to which the battery is connected. The charging and discharging equipment is generally referred to as power conversion equipment. The number of strings in parallel is determined by the capacity, i.e., the number of ampere-hours, or the energy rating, i.e., the number of watt-hours, that is desired by the user of the battery system.
In order to standardize the terms used herein,
While the term `finish charging` is well-understood by both battery manufacturers and users, `equalization` is not. In fact some authors incorrectly use the terms interchangeably, and in other writings there is some confusion over the terms. Here, finish charging is defined as a process at the end of nearly every bulk charge when the battery has reached regulation voltage and the charge current tapers (i.e., reduces in magnitude) because of an exponential increase in the effective resistance of the cells of the battery as further charge is applied. Finish charging a lead-acid battery typically takes on the order of one to four hours.
`Equalization` has two distinct definitions in the battery field. For one definition, which is not utilized in this invention, it means actively adjusting the charge of individual cells in a string in order to restore each cell to an equal state of charge. For the other definition, which is utilized herein, equalization in a lead-acid battery is accomplished by an extended-period, relatively low current, charge following a regular recharge. During equalization, the voltage is raised a little above the fully-charged open circuit voltage and current is limited for a period of on the order of twelve to twenty-four hours. The current drops during the early part of the equalization process and for most of the process is typically a few percent of the normal charging current. Thus, the electrical power required for equalization is a small fraction of the power normally required for charging.
Finish charging and equalization of lead-acid batteries, and the complete discharge of a nickel/cadmium or zinc/bromine battery are all examples of battery management procedures, as defined in this invention, that, if properly done, will help maximize the life of the battery, but are not necessary for satisfactory short-term operation.
A specific example of an electric system for which the current invention may find utility is a solar hybrid system. Solar hybrid systems are increasingly used to provide power to electricity end-users at locations that are remote from the transmission and distribution systems of utilities. The design of a solar hybrid system is much like the generic electric system shown in
Lead-acid batteries are frequently used in solar hybrid systems. Each battery system consists of a number of 2 volt cells, or 6 volt (three-cell) or 12 volt (six-cell) modules, connected in series to form a string having a useful output voltage. Most solar hybrid systems use a plurality of cells or modules in a series string to provide a sufficient output voltage. In order that the battery system has adequate capacity to cover relatively long periods without solar energy and without having to turn on the generator (for example: long winter nights) most solar hybrid systems use a plurality of strings in parallel.
The operation of a typical solar hybrid system is as follows: on a sunny day, direct current (DC) from the photovoltaic array 12 (the PV) is provided to the power conversion equipment 14 and 26, which may convert it to a different DC value and then to alternating current (AC) to power the load, or the excess energy at the different DC value may be used to charge the battery. If there is not enough solar energy to generate sufficient output from the PV, or at night when the sun is down, the load 60 is supplied with energy from the battery 1. When the battery 1 needs charging, the generator 10 supplies the load and recharges the battery 1. The generator 10 is turned off when the battery 1 is filly charged. From an operating maintenance cost standpoint, energy from the PV 12 is least expensive; supplying energy to the load from the storage battery I is more expensive because of the inefficiency of the battery and because use degrades the life of the cells; and operation by generator 10 is most expensive since a suitable high-grade fuel must be provided and the generator requires periodic maintenance. Furthermore, utilization of a generator at a small fraction of its power capability is particularly expensive since a generator is often inefficient under this operating condition and requires more maintenance.
During discharge mode, if solar energy is not available, the battery 1 (Strings A and B) provides the power for the user's electrical loads 60 via power conversion equipment 26 that converts DC to AC. If solar energy is available, the PV 12 may provide some or all of the power for the user's loads, and at times, the PV may be providing more power than needed by the customer, so that the battery becomes partially recharged even though in the discharge mode. When the battery voltage reaches a preset lower level, as measured by the power conversion equipment, the generator 10 is started so that the battery can be recharged. However, since all the strings of the battery are connected in parallel in current systems, as illustrated in
For much of the charge time with the AC source (the generator) 10, i.e., the bulk charging period, the lead-acid battery of this example (as with batteries based on other chemistries) can accept charge efficiently at almost any power level that can be provided by the source of charging energy. However, towards the end of charge, the effective resistance of the battery (defined as the ratio of the excess voltage required to pass the charging current to the charging current) increases and the efficiency of recharge (the fraction of the current being applied that increases the real state of charge of cells) decreases. The power conversion equipment 20 (acting as an AC to DC converter during battery charge) is set to reduce the charging current near the end of charge so that the charging inefficiency does not become too large. Charging is terminated when the charging current reduces to a preset lower limit, but it is not allowed to proceed for too long since the. generator is not efficient when the power is being produced below it's rated value. As a result of this termination criteria, charging is usually not completed to a level recommended by the battery manufacturer, and the battery must be equalized periodically in order to ensure that capacity is not permanently lost. Any solar energy that is generated by the PV during finish charge or equalization by the generator cannot be used effectively and is lost, leading to further inefficiency. As a consequence of all these factors, more fuel is used by the generator than if the battery did not require finish charging and equalization, and the generator must be subjected to maintenance more frequently because it runs for a long period of time at low power. In addition, the equalization process itself and any failure to frequently complete finish charging both lead to a shorter life for the battery than would be expected under optimal charging conditions.
The deleterious effects of sub-optimal charging and discharging, as described above, are exacerbated by the fact that neither individual cells nor individual modules are identically constructed, so some cells and modules: 1) accept charge more efficiently during the finish charge; 2) discharge at higher voltage; as compared to other cells and modules; 3) some cells deteriorate at a faster rate than other cells. Since one bad cell may cause a battery to fail, it is desirable for efficient use of a battery system that the battery be charged and discharged in an optimal way.
The definition of equalization and finish charging frequently discussed in other patents is not the same as that used in the current document.
A more thorough explanation of the need for battery equalization may be found in U.S. Pat. No. 5,504,415 of Y. Podrazhansky et al., which patent discloses a system for equalizing individual batteries in a series string of batteries by shunting charging current around cells based on cell temperature. According to our definitions, the process of this patent would be called finish charging.
U.S. Pat. No. 5,905,360 of S. Ukita discloses an equalization system for a hybrid vehicle which uses a generator to equalize all modules in a series string, and then uses fully charged modules in the string to transfer charge to less fully charged modules. The load is not powered by the battery while this transfer is occurring, thus the battery system is not available to the load during the equalization procedure. Again, the process being accomplished here is what we would call finish charging.
Another system for equalizing a battery is shown in U.S. Pat. No. 6,150,795 of N. Kutkut et al, where battery charge equalization is carried out utilizing modules connectable in staggered relation between pairs of batteries in a series connected string of batteries. The device disclosed in this patent is commercially known as PowerCheq™, a product of PowerDesigners, LLC of Middleton, Wis.
It is an object of this invention to provide a method for optimally charging and discharging multi-string batteries in electric systems so that battery life is maximized and the performance of the system of which the batteries form the storage component is also maximized.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, this invention is a method of optimally managing battery strings in an electric system for continuously powering the load, with at least two battery strings for selectively powering the load, and an electrical AC source for selectively powering the load and charging the battery strings. The method includes the processes of charging mulitple battery strings with the AC source; management of one battery string while the other battery strings power the load and without using the AC source; powering the load with all battery strings, without using the AC source, until discharge is deemed optimally complete; bulk charging all the battery strings using the AC source; and the sequential management of each battery string while the remaining battery strings power the load and without using the AC source. Alternately, the method includes the processes of charging multiple battery strings with the AC source; the sequential management of each battery string while the other battery strings remain available to power the load if necessary. As yet another alternative, the processes of charging multiple battery strings with the AC source; powering the load with all battery strings, without using the AC source, until discharge is deemed optimally complete; management of one battery string while the other battery strings are being charged using the AC source; completing the charge of all battery strings with the AC source; and then sequential managing each of the battery strings. Management may include finish charging, equalization, or total discharge, depending on the situation and battery type.
The accompanying drawings, which are incorporated in and form part of the specification, illustrate the present invention and, together with the description, serve to explain the principles of the invention.
The method by which batteries are optimally charged and discharged consists of several sets of processes, with different battery chemistries requiring different sets of processes, together with the equipment for implementing the processes. The processes involved in practicing the method are shown in FIG. 3.
For lead-acid batteries in hybrid power systems for which frequent finish charging is advantageous, and other batteries with similar characteristics, the set of processes A, B, and C (shown as a sequence of solid lines in
For nickel/cadmium batteries in uninterruptible power supplies, in which the battery is kept fully charged most of the time but infrequent complete discharges are necessary for maximizing battery life, and similar batteries and systems, the set of processes E and F (dotted lines in
For zinc/bromine batteries in applications where the battery is frequently deep cycled, as in the case where the battery is used for matching the load and the supply in electric systems with a generator, such that infrequent complete discharges are necessary for maximizing battery performance, and similar batteries and systems, the set of processes C, D and F (dashed lines in FIG. 3 and solid line process A) are applicable. The processes D and F (dashed lines) are only applied occasionally, as required, with the battery strings being discharged (process C) and recharged (process A) without use of the invention at other times. Process D involves completely discharging one of the strings of the battery while recharging the other strings using process A. Process F involves recharging the string that has been completely discharged and completing the charge of the remaining strings as necessary.
The implementation of the foregoing sets of processes will now be described.
For the purpose of this patent, the description will be based on the individual components in order to more easily explain the operation of the invention. However, any component or combination of components known to those of ordinary skill which performs the functions identified for these components may be utilized in the practice of the invention. The equipment described above represents currently practiced art for electric systems. The additional components described below are utilized in the practice of this invention.
Switch 24 is connected to the input 32 of a DC/DC converter 30 and to the contacts 42a and 44a of single pole, double throw switches 42 and 44, respectively. Contacts 42b and 44b are each connected to output 34 of converter 30. Switches 42 and 44 are connected, respectively, to battery Strings A and B, which together constitute the battery. Converter 30 transforms the DC voltage from one battery string to a higher voltage for finish charging the other string, as set forth hereinafter. The switches 42 and 44 may be either electromechanical relays or switches built from combinations of semiconductors, or other known devices. The use of combinations of semiconductors rather than relays allows the magnitude as well as the direction of the current to be controlled. It is the interconnection of these components, and their use in combination as set forth hereinafter, that is the invention.
The example of
A battery controller 80, which is part of the current invention, receives inputs representative of the voltage V, current I, and temperature T, of each battery string; and provides output commands to set each of switches 42 and 44. The implementation of this control system with a microprocessor and controllers, or other equivalent equipment, to accomplish the goals of the invention, as set forth hereinafter, is a routine matter for one of ordinary skill in the art.
The charge continues with the system in the state shown in
It is one of the objects of the current invention to minimize the period of finish charging with the AC source so as to maximize system efficiency. This can be done without deleterious effect on cell life because of the finish charging method which is shown in FIG. 5.
Converter 26 now acts as a DC-AC converter, so switches 22, 24 ate set to the discharge (`d`) position, switch 42 is set to 42b position, and load 60 is powered by one battery string (String B, as illustrated) through contact 44a and DC-AC converter 26. Switch 15 has opened and AC source 10 is not being used. The other battery string (String A, as illustrated), is finish charged in process B, by power from string B, as voltage from String B is up-converted or boosted by the DC-DC converter 30 and applied through contact 42b to String A. In this manner, String A receives a lengthy finish charge from String B, without the inefficient use of AC source 10.
Finish charging continues with the equipment configured as shown in
After discharge is seemed complete, the system returns to the process A configuration of FIG. 4. However, after both battery strings have been charged and the system switches to the embodiment of
While the embodiments of this invention show two battery strings A and B, it should be understood that the invention may be practiced with any number of parallel battery strings. With more than two battery strings, the system operates on the string being managed in the manner described herein, while a subset or all of the remaining battery strings perform the function described herein for the string that is not being managed. That is, if string A is being managed, in a multi-string embodiment all strings except string A could provide power. And while the two string embodiment has each string being managed (e.g. finish charged) on alternating cycles of the system, a multi-string embodiment could first manage string A, and then manage a different string on each succeeding cycle until all strings had been managed, before managing string A for the second time. Alternatively, if there were a sufficient number of strings to provide management power, two or more strings could be managed successively during each cycle. Furthermore, there may be instances where it is desirable to manage one string more often than other strings. Also, with a sufficient number of strings, one string could be finish charged by a second string while the remaining strings power the load. A constant in all such scenarios is fact that the primary AC source is not used for string management, and that the load is powered by battery strings while other strings are being managed.
An alternative method for accomplishing finish charging electric systems is shown in FIG. 7. In this Figure, finish charging is accomplished with a separate AC-powered battery charger 61 which eliminates the need for the DC-DC converter 30. In charge mode this alternative operates in the same way as the example shown in
It should be readily understood that this invention is easily expanded to cover more than two battery strings. In that case, two or more strings could function in parallel as the charging string in
For solar or other renewable hybrid systems with lead-acid batteries, such as that shown for process B in
As another example of the utility of our invention, the method illustrated in
Furthermore, if batteries are used that periodically require a complete discharge in order to maintain their maximum efficiency, such as zinc/bromine (stripping) and nickel/cadmium (erase memory), then the circuit shown in
The benefits that can be provided by practice of this invention have been demonstrated in two ways: by simulations using a computer model and by measurements on a battery storage system that includes the equipment needed to practice the invention.
The invention was simulated with a computer model using the characteristics of one of the lead-acid battery types that has been incorporated into developmental solar hybrid systems, and using the fuel consumption characteristics of a generator that has been used in a solar hybrid system field test. The model did not include a simulator for a solar PV array since the availability of solar energy is unpredictable and therefore difficult to model. Rather, the model was for a generic electric system of the type illustrated for processes A, B and C in
A group of experiments to demonstrate the benefits that may be realized from practice of this invention were performed with the equipment shown in FIG. 10. This consisted of two strings of modules, String A and String B, each with two Trojan T105 6 V battery modules with a rated capacity of 225 ampere-hours. The two strings were connected via semiconductor switches 42c and 44c to a 12-15 V, 40 amp battery charger acting as an AC-to-DC converter 20; via semiconductor switches 42d and 44d to a bank of resistors connected to ground, which thereby acted as load 60, so as provide a sink for current of 26 to 30 amps, depending on string voltage; and via semiconductor switches 42f and 44f to a 12 V DC to 15.5 V DC converter to provide power for finish charging from one string to the other. In
During the finish charge of String A which continued for approximately three and one half hours until 15:35 hours, 10% more capacity had been returned over that discharged from String A in the preceding discharge, the current continued to taper down, indicating that the String A had not been fully recharged in the preceding bulk charge period. The change to discharging both strings at 15:35 hours is also obvious in
An investigation and evaluation of plots similar to that shown in
The applicability of these advantages to solar hybrid systems, which typically use lead-acid batteries as the energy storage element, would be obvious to those skilled in the art. However, there are other systems that either are required to or would benefit from operation for long periods or continuously at an intermediate state of charge. For such systems, the invention allows for optimal charging and discharging even within the limitations of the customer's needs for discharge power or the timely availability of power for recharge.
For lead-acid batteries, completing charges with a finishing charge relatively frequently is advantageous in extending the life of the batteries. For other types of batteries, for example zinc/bromine batteries or certain nickel/cadmium batteries, fully discharging individual strings relatively frequently is necessary. This currently recognized benefit cannot be quantified at present. Nevertheless, as discussed above, the method of our invention can be used for optimal charging and discharging of batteries based on chemistries other than lead-acid.
The particular sizes and equipment discussed above are cited merely to illustrate a particular embodiment of this invention. It is contemplated that the use of the invention may involve components having different sizes, capacities, and shapes as long as the principle of optimally charging and discharging the battery even when the power requirements of the application do not alone permit this is followed. For example, a 20 MW battery system in Puerto Rico, with 6 strings of 1000 cells in a series in each string, is operated so as to provide frequency regulation and rapid reserve for that island's electric system. In this application, the battery must be held at an intermediate state of charge for most of the time. A lead-acid battery has been used as the energy storage component of the Puerto Rico battery system. The battery in this situation would have benefited from the method of our invention, since it would have allowed more optimal charging and discharging without interfering with the customer's requirements for discharge and capabilities for recharge. Clearly, much larger capacity switches, and measurement transducers appropriate to the size of the battery would be required for this application, but the switches and the data would be managed in the same way as described above.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the preceding description or may be learned by practice of the invention. The applicability of the invention to storage batteries based on other chemistries, and for other applications in which there is an opportunity for optimally charging and discharging may also be apparent upon examination of the following to those skilled in the art. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
Symons, Philip C., Atcitty, Stanley, Butler, Paul C., Corey, Garth P.
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